1. Field of the Invention
The present invention relates to an image display medium, a process for forming images, and a multicolor image-forming apparatus. More specifically, it relates to an image display medium, a process for forming images, and a multicolor image-forming apparatus that can repeatedly form color images upon light irradiation.
2. Description of the Related Art
Erasable or rewritable image display media using photochromic compounds that can reversibly change their colors upon irradiation with light have been proposed. However, practical processes and apparatus that can repeatedly rewrite full-color images have not yet been proposed.
For example, Japanese Patent Application Laid-Open (JP-A) No. 05-271649 discloses, as a process for forming multicolor images using a photochromic compound, a process of applying ultraviolet radiation with distinct corresponding wavelengths to a mixture of three diarylethenes, where the diarylethenes include one turning golden yellow upon irradiation with ultraviolet radiation wavelength of 254 nm, one turning red upon irradiation with ultraviolet radiation wavelength of 313 nm, and one turning bluish purple (violet) upon irradiation with ultraviolet radiation wavelength of 365 nm.
To form full-color images, three or more photochromic compounds developing three primary colors (blue, green, and red, or yellow, magenta, and cyan) must be controlled by light to develop and/or reduce their colors. The aforementioned process requires that wavelengths of the ultraviolet radiation determine whether or not each of the material compounds develops its color. In other words, the process requires three or more photochromic compounds showing absorption in ultraviolet wavelengths, where each of the wavelengths does not overlap one another, and that these photochromic compounds show the three primary colors upon color development. However, no compounds comprising such compounds has been found in reality. To use in practice, materials must have satisfactory repetition durability, heat and moisture resistance in addition to color developing properties. Such materials satisfying all the requirements are very difficult to develop.
Japanese Patent Application Laid-Open (JP-A) No. 07-199401 proposes a process for forming color images. In this process, ultraviolet radiation of 366 nm is applied to three photochromic fulgide compounds turning yellow, magenta, and cyan upon color development, respectively, to thereby make all the fulgide compounds to develop their colors. White light is then applied to the fulgide compounds through a color positive film to selectively reduce the colors of individual photochromic fulgide compounds according to necessity to thereby form color images. This process only requires one ultraviolet light source, but requires a preparation of a color positive film of the target image upon each use and is not practical for use in color image output in office systems.
These and other proposals for rewritable image media and processes using photochromic compounds are not satisfactorily practical in the formation of color images.